Quantum-enhanced Network Tomography
For network operators, this work provides a method to improve network monitoring accuracy using quantum-enhanced probes, but the approach is incremental as it applies known quantum estimation techniques to a specific network tomography problem.
The paper proposes using quantum probes (coherent-state pulses with squeezing or weak entanglement) to estimate link transmissivities in optical networks, and develops a probe construction algorithm that maximizes Fisher information. The method shows quantum improvement in estimation accuracy compared to classical approaches.
Network tomography refers to the use of inference techniques for inferring internal network states from end-to-end probes. Quantum probes, implemented by sending blocks of $n$ coherent-state pulses augmented with continuous-variable (CV) squeezing ($n=1$) or weak temporal-mode entanglement ($n>1$) over a lossy channel to a receiver with homodyne detection capabilities, are known to carry information about the channel transmissivity. Assuming a subset of nodes in an optical network is capable of sending and receiving such probes through intermediate nodes with all-optical switching capabilities, we leverage these quantum probes to estimate link transmissivities. To determine how to route the probes in a network, we propose a probe construction algorithm that guarantees link identifiability, while maximizing the number of information orthogonal sets of transmissivities. A set of probes induces a Fisher information matrix (FIM). We then derive two metrics, the determinant of the FIM and the trace of its inverse, to evaluate the performance of the probes. In particular, our results can be used to characterize the quantum improvement in estimating link transmissivities in a general optical network.